To firmly connect sheet metal parts, in industrial plants, such as for example automobile manufacturing plants, welding robots with welding guns are often employed. During a welding operation, the welding gun presses two or several sheet metal parts against each other from two opposite sides while electric current flows through spot welding electrodes mounted in the gun arms of the welding gun, so that the sheet metal parts are welded to each other with utmost precision. The closing force of a spot welding gun during the welding operation can add up to 8 kN. After approx. 100 to 300 welding operations, the spot welding electrodes are worn down or coated with welding residues to such an extent that reliable welding is no longer possible.
Usually, the welding electrodes comprise exchangeable caps. However, they can also be made in one piece without such caps. The following explanations likewise concern both cases as the milling cutter, the milling device and the apparatus can be equally employed for one-piece electrodes and for electrodes with caps.
To return the worn down spot welding electrode tips again to their original shape, said tips are subsequently machined by means of an electrode milling cutter. To repair the worn down spot welding electrodes that consist of relatively soft copper, it is in most cases sufficient to remove less than 0.1 mm of material from the spot welding electrode tips.
With electrode milling cutters as they are known from prior art, however, double or triple amount of this thickness (0.2 mm to 0.3 mm) is removed in practice. On the one hand, this leads to the copper electrodes being shortened to an unnecessarily great extent, so that the electrodes or the electrode caps must be completely replaced correspondingly earlier. On the other hand, the unnecessarily substantial removal also leads to a corresponding increase of the amount and size of the removed copper chips, thus increasing the risk of the milling cutter getting clogged. The replacement of spot welding electrodes or the cleaning of an electrode milling cutter usually results in the complete welding operation having to be interrupted for quite a long time, leading to considerable follow-up costs due to loss of production or rejects being produced.
To reduce the material removal during the milling of the spot welding electrodes, it would be conceivable to have the welding guns, which grip with the spot welding electrodes into the electrode milling device for milling off, grip into the electrode milling device either shorter or with a lower force of pressure. In practice however, it is difficult to realize such a procedure as the closing time of spot welding guns can be hardly reduced to below 0.7 s and thus milling time cannot be shortened arbitrarily. It is equally difficult to clearly reduce the closing force of 1 to 2 kN as this would result in positioning inaccuracies, such as e.g. the welding arms bending less at a reduced force of pressure, whereby the orientation of the active areas with respect to each other changes.
It would be furthermore conceivable to reduce the amount removed during milling by selecting the speed of the electrode milling cutter to be lower than the usual 200 to 700 rpm, or by selecting the tool clearance at the milling edge or edges of the milling cutter to be smaller than the 6° usual for nonferrous heavy metal. Both of these possible modifications, however, would lead to clearly worse milling results, such as irregular material removals, vibrations during the milling operation or a poor machining result.